This invention generally relates to communication systems, and more particularly, to a scheme or protocol for releasing busy slots in a slotted bus network.
Various types of communications networks use fixed slots as basic units for information exchange. Such networks include the distributed-queue dual bus (DQDB) network. In such networks, an empty slot is used by a node to carry information to a specific destination, or to several destinations in case of multicasting. Several protocols have been proposed to control the use of empty slots in a way that attempts to ensure, among other features, fairness among nodes.
Developers of several slot-base protocols attempt to increase the effective throughput of the network by allowing the reuse of slots after their payload is delivered to the intended destination. This desired feature can be accomplished in different ways, depending on the network topology and the acceptable level of complexity of the access control schemes.
The spectrum of slot reuse schemes can be broadly classified into four categories referred to as (i) no slot reuse, (ii) source slot release, (iii) destination slot release, and (iv) specialized erasure nodes.
The no slot reuse scheme is simple but inefficient and is mainly used in open bus topologies. This scheme does not provide for any slot reuse, and a used slot propagates to the end of the network with its busy indicator on or set. The disadvantage of such a simple scheme (which might be better referred to as the absence of any slot reuse scheme) is that some network bandwidth is not used while it could have been used.
In the source slot release scheme, which is mainly used in networks having ring topologies, a node is required to release the slots that it uses after the slots make a full revolution around the network. The principle behind this release scheme is that a released slot carries information that has definitely been received by the intended destination or destinations, since the slot has gone by every node in the network. Therefore, these slots can be safely released and subsequently reused to carry additional segments of information. The disadvantage of such a very conservative release scheme is that some network bandwidth is wasted in carrying slots uselessly all the way from the destination nodes back to the source nodes. Moreover, every node must delay every slot passing the node until the node determines whether it is the source of information carried in that slot, in which case it has to reset the busy indicator of that slot. This delay may be considerable because of the size of the fields that need to be examined.
In the destination slot release scheme, every destination node is allowed to turn off, or reset, the busy indicators of all the slots actually received by that node. This scheme makes the best possible utilization of the network bandwidth; however, this release scheme introduces additional processing delays at every node, similar to the delays involved in the source slot release scheme.
The fourth type of slot reuse schemes, specialized erasure nodes, has been recently proposed to avoid or minimize any delays at regular network nodes; and this is done by using specialized nodes, rather than the regular network nodes, to turn off the busy indicators of previously received slots. To achieve this, a destination node has to indicate that a given slot has been received, and the erasure node then turns off the busy indicator of that given slot. Under this scheme, the delays needed to check each slot to determine whether that slot has been received by its destination node, occur only at the erasure nodes and not at every node in the network. The use of specialized erasure nodes introduces an unwanted fairness problem, though, because the quality of service provided to the two nodes lying on either side of each erasure node differs significantly.